1. Field of the Invention
The present invention relates generally to the field of circuit interrupters and control circuits therefor, and more particularly, to an improved current-sensing arrangement and method for providing a signal that suitably represents line current over a wide current range.
2. Description of the Related Art
Arrangements for sensing the current in an AC power line and providing a signal representing the current are shownn in U.S. Pat. No. 4,434,415 and in co-pending, commonly-assigned U.S. patent application Ser. Nos. 506,942 (now U.S. Pat. No. 4,571,658), 658,239, (now U.S. Pat. No. 4,642,724) and 506,944 (now U.S. Pat. No. 4,571,658) filed in the name of J. W. Ruta. For example, FIG. 1 of U.S. Pat. No. 4,434,415 illustrates a current transformer 46 positioned coaxially about a conductive member through which line current is passed. As further disclosed in the aforementioned patent applications, the current transformer is utilized to provide an output signal for use by control circuitry to provide appropriate trip signals in accordance with predetermined time-current characteristics.
As set forth in application Ser. No. 791,195 filed on Oct. 25, 1985, where the control circuit is required to operate over a wide current range, e.g. 400-40,000 amperes rms, the use of an iron core transformer for this wide current range is not practical since the output signal no longer represents the current as the transformer begins to saturate.
Some control arrangements utilize two transformers which function independently of each other. For example, U.S. Pat. No. 4,297,741 utilizes an iron core transformer for providing current signals to long-time and short-time trip circuits while an air core transformer is utilized to develop signals representing the rate of change of the current, di/dt, as an input to an instantaneous trip circuit.
A digital electronic current transducer is disclosed in an article entitled "Digital EHV Current Transducer" by L. B. Berkebile, 1980, IEEE, 80 SM 647-8. A line potential encoder unit is also disclosed and two types of current-driven power supplies are discussed: a saturable current transformer; and a controlled current-switching current transformer.